This work is concerned with the in vivo hormonal regulation of follicular and luteal growth and regression. Representative projects include: a. Hormone levels in the acute and chronic phases of unilateral ovariectomy of cyclic hamsters. b. Hormone levels - (progesterone, estradiol, FSH, LH and prolactin) in superovulating hamsters. (Induced by single injections of pregnant mares serum (PMS). c. Effects of superovulation on the course of pregnancy and hormone levels in hamsters. d. Associated changes in luteal morphology and hormone levels following estrogen induced luteolysis in hamsters. e. Can estrogen act as a direct luteotropic hormone in hysterectomized-hypophysectomized pregnant rats? Bibliographic references: 1975 Ridley, K. and G.S. Greenwald. Progesterone levels measured every two hours in the cyclic hamster. Proc. Soc. Exptl. Biol. Med,: 149: 10-12. 1975 Siegel, H.I. and G.S. Greenwald. Prepartum onset of maternal behavior in hamsters and the effects of estrogen and progesterone. Horm. Behav., Vol. 6: 237-245.